This invention relates generally to a novel spool valve off-center feedback spring arrangement having specific utility in the servovalve art. More particularly, this invention concerns an improvement in servovalves having spool-valve-position feedback provided by a feedback spring, wherein the feedback spring is not aligned vertically with the center line of the spool valve. In the past it has been common to sense spool position in hydraulic spool valves by providing a mechanicallinkage in the form of a feedback spring in contact with a portion of the spool. As the spool moves the feedback spring typically pivots about a pivot point and provides a feedback signal to, e.g., a torque motor, which in turn is controlling the direction of hydraulic fluid to one side or the other of the spool, causing the spool valve to move a desired amount in a desired direction.
The feedback spring typically has a ball attached to the end of the feedback spring, with at least portions of the surface of the ball contacting the spool. In one prior art arrangement, wherein the feedback spring cannot be positioned, or is undesirably positioned, so that the longitudinal axis of the feedback spring, in the vicinity of the spool, is aligned with a cross-sectional diameter of the spool, a groove is typically formed around the circumference of the spool having a width equal to the diameter of the ball. The ball then rests in the groove and as the spool moves the feedback spring thus is also caused to move in the same direction. The disadvantage of this arrangement is that the contact between the inner walls of the groove and the surface of the ball is a point contact on each side of the wall. Point contact, coupled with high feedback spring loads, accelerates the wear to both the surface of the groove and the surface of the ball at the point of contact, ultimately causing an improper feedback signal when the ballgroove interface clearance is increased.
In addition, in this type of arrangement the spool will typically tend to rotate around its longitudinal center line axis, thus exacerbating the wear problem described immediately above.
One manner of solving the wear problem is shown in the patent to Cobb, U.S. Pat. No. 3,584,649. FIG. 2 of that patent shows a feedback spring arrangement in which the ball extends into a hole in the spool. This eliminates the point contact problem since the ball and hole have the same diameter and also eliminates the spool rotation problem. However, this arrangement can only be used when the center line of the hole corresponds with a cross-sectional diameter of the spool and the feedback spring extends into the hole generally along the center line of the hole.
Further, in many spool valve applications it is impractical or undesirable to extend the feedback spring downwardly into a hole in the spool, which hole has a center line corresponding with a cross-sectional diameter of the spool, and where the feedback spring extends generally along the center of the hole. Thus, it is often more desirable to employ an off-center feedback spring, which prior to the present invention resulted in the necessity to accept the undesirable effects of wear on feedback spring ball noted above, due to point contact between the feedback spring ball and the walls of the groove in the spool, and rotation of the spool valve.
It can therefore be seen that while such prior art arrangements have exhibited some degree of utility in accomplishing the desired feedback responsive to the motion of the spool valve, some room for significant improvement remains.
The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known feedback spring arrangements. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that feedback spring arrangements appearing in the art have not been altogether satisfactory.